In recent years, in the field of automobile paints for example, problems have arisen with a loss of quality of the appearance of paint films due to chemical and physical factors such as etching with acid rain and scratching when washing the paint film for example. In order to resolve these problems, acrylic-melamine based top coat paint compositions into which soft segments have been introduced by the conjoint use of .epsilon.-caprolactam modified acrylic resins, specified low molecular weight polyols and alkoxy monomeric melamine have been suggested as essential structural materials for paint films (for example, Japanese Patent Kokai 6466274).
However, with these paint compositions, the paint film is softened because the paint film forming resin itself has been modified with caprolactone and the scratch resistance can be improved, but the acid resistance is reduced because alkoxy monomeric melamine is used, and this is unsatisfactory for dealing with today's severe acid rain.
Furthermore, top coat paint compositions in which .epsilon.-caprolactam modified acrylic resin and non-yellowing type block polyisocyanate compounds are essential components have been suggested for the same purpose (for example, Japanese Patent Kokai H2-305873).
However, although both the scratch resistance and the acid resistance are improved with these paint compositions, the paint films tend in fact to yellow even though only to a slight extent because of the use of the block polyisocyanate compounds as crosslinking agents, even though they are said to be of the non-yellowing type.